I extracted this information from George Dockray off of the GuzziTech website after he made note of it on the Yahoo! Loopframe_Guzzi news group. In George's own words:

Here's what I've found out about Loop / Eldo advance curves as they relate to detonation problems.

1st off, I noticed in the owner's handbook, that 98/100 NO R.M. is the recommended fuel rating. I poked around, but can't find anything that might be a clue as to what the NO part of that is, the R.M. part is plain enough, so I assume that it's the same expression for octane rating as is used in N America now unless somebody knows different. The highest octane rating usually found is 94 R/M in Canada and 91 R/M in the US Pacific NW. Especially in the US PNW then, the difference from the factory recommended octane rating and what's available is considerable. So, I'm taking that as pretty good probable cause exhibit A.

I have an Eldo distributor off the bike (thanks Steve) to install on the piece of steam-punk apparatus you can see below. Somebody was cleaning out their garage and it ended up given to our shop. Put a charged-up 6V battery in there and the damn thing worked! The distributor spec paper scroll in it includes up to 1956, so there you go.

Distributor advance curve tester.

Photo courtesy of George Dockray.

Anyway, I installed the Eldo distributor to this rig and ran it up with my note pad and then drew up the 1st hand-drawn graph below as compared to the graph in the service manual just above it. The total advance is right, but it's not quite the pair of straight shots from the factory graph. That's not surprising, but it's really not that far off. On the actual stock curve graph you can see that just pulling on the 1 spring as the stock spring set-up is designed to do, the 1st part of the curve is pretty steep - you're already at 20° BTDC @ 2000 RPM. So there's a second reason why stock Eldos might be prone to detonation - lots of advance early in the engine RPM range.

Moto Guzzi Eldorado advance curve from factory publications.

Photo courtesy of George Dockray.

Observed advance curve of the Eldorado distributor using the tester.

Photo courtesy of George Dockray.

The bottom graph is one of the 3 curves available with Cliff Jefferies REC-Ignition module. I have not included the other 2 since after drawing them out based on Cliff's curve numbers he sent me, it looks like the most appropriate one.

The REC-Ignition curve is quite a bit shallower and even given an 8° BTDC static point rather than the 5° stock setting, you have (for instance) a 12° spark point at 2000 RPM rather than 20° and shallower all the way up the curve until you get to about 3200 RPM - a point where you have a high enough engine speed that will not be as prone to detonation except for very high load situations. The REC-Ignition web page mentions that you cannot use points as the trigger signal with the built-in curves. I'm partial to the Laubtec unit rather than the Harper's / Pertronic for a couple reasons, but that's another subject.

So there it is. Granted, a fixed curve set-up is not a silver bullet to be sure - humping it up a long hill in E Wash. in the summer when it's 92° F outside pushing a windshield and saddlebags at 70 MPH with all your gear piled on it, but a reasonable band-aid that should help address a tendency towards detonation with an otherwise stock set-up, especially for those that would like to retain a visually-identical to stock distributor.

BTW: I forgot to mention that the REC-Ignition has selectable ignition coil charging soak times so it should do a far better job making efficient use of the coil and give easier starting, and perhaps off-set the still less than perfect advance curve to a certain extent in the way of fuel consumption.